Hello Folks!
My LED application is supposed to signal OK with a green (3mm) LED and "not OK" with a red LED. The two LEDs are in different parts of the circuit and don't have common ground or supply.
I'm looking for a green and a red LED each providing the about same subjective brightness, but don't know which brightness (in mcd) to choose.
I'm also somehow missing a diagram in LED datasheets telling me the typical brightness vs. Diode current or/and voltage which would also be helpful in judging circuit sensitivity.
Any help enabling me to choose or proposal of specific LEDs for the purpose is thankfully appreciated!
Greetings,
Winfried
My LED application is supposed to signal OK with a green (3mm) LED and "not OK" with a red LED. The two LEDs are in different parts of the circuit and don't have common ground or supply.
I'm looking for a green and a red LED each providing the about same subjective brightness, but don't know which brightness (in mcd) to choose.
I'm also somehow missing a diagram in LED datasheets telling me the typical brightness vs. Diode current or/and voltage which would also be helpful in judging circuit sensitivity.
Any help enabling me to choose or proposal of specific LEDs for the purpose is thankfully appreciated!
Greetings,
Winfried
Since LED sensitivities have varied - mainly because newer ones are a bit brighter with less current through them, I go "old school" to configure the circuit.
I determine the actual DC supplied from the circuit first, then using a variable power supply set that voltage. - then choose the LED, and use a simple 10K potentiometer in series with it and a 100 ohm resistor, and adjust the pot for the desired brightness.
Then, I measure the pot's resulting value, add the 100 ohms, and use that value for a simple resistor close to that value.
Works fine, every time.
I determine the actual DC supplied from the circuit first, then using a variable power supply set that voltage. - then choose the LED, and use a simple 10K potentiometer in series with it and a 100 ohm resistor, and adjust the pot for the desired brightness.
Then, I measure the pot's resulting value, add the 100 ohms, and use that value for a simple resistor close to that value.
Works fine, every time.
I would agree with the above. You will never get the brightness you want by trying to calculate it based on published figures. Any LED always has a resistor in series with (well in 99% of simple applications) and so you just scale the resistor to suit.
I would suggest you buy the red and green from the same manufacturers range though as LED's vary in physical (internal) appearance and so are likely to look better if from the same brand and range.
A modern high brightness LED can be blindingly bright with just a few milliamps... think of all those cycle lamps you see flashing away. I have some high efficiency ones that give faintly detectable visible light when fed from a 9 v battery and 10meg series resistor. Amazing.
I would suggest you buy the red and green from the same manufacturers range though as LED's vary in physical (internal) appearance and so are likely to look better if from the same brand and range.
A modern high brightness LED can be blindingly bright with just a few milliamps... think of all those cycle lamps you see flashing away. I have some high efficiency ones that give faintly detectable visible light when fed from a 9 v battery and 10meg series resistor. Amazing.
Most modern LEDs are quite sensitive, including "normal" ones.
Generally, a current of 1 to 5mA is quite sufficient for normal environments.
At this level, a high brightness type will be blindigly strong.
Anyway, if you are not satisfied with the light output, you just need to change a resistor (generally)
You don't need a diagram: for modern LEDs, the light output is a very linear function of the forward current for their useful range (and beyond): if you have one pair of values, you can compute the mcd/mA constant, and use it to extrapolate the intensity at any current.
Most LEDs types are binned according to their intensity for a given current.
Choosing the right group will help you narrowing down the initial dispersion on the intensity.
Note that some subjectivity is involved in the perceived brightness, and this may vary between individuals and be influenced by the visual context.
The rest is simply a question of ohm's law and simple arithmetics
Remember also that doubling the intensity does not mean that the eye percieve it as a twice as bright. Its like sound, logarithmic.
Hi friends,
thanks for all the of feedback and the valuable implementation hints!
Let me be a bit more specific on the reason for my question:
The red LED shall already clearly visibly light up with a few mA (maybe 5mA, maybe less?) of current, that's what I meant by "sensitivity". I don't know which specific type this may provide and I don't have LEDs in the drawer at all to try out which one would fit and therefore would like suggestions what to buy. Maybe that's possible...
So, the red LED is the more ecritical one, as it indicates failure drivern by the output transistor of an SFH6206 optocoupler and should visibly light up with as little current into the OC Diode input as possible. To achieve that I have made provisions on the board for an additional optional transistor forming a darlington with the output NPN of the OC if necessary. The green LED indicates OK and could be adjusted in brightness as advised, i.e. less critical.
Hope the above makes good sense of what I'd like to do...
Greetings,
Winfried
thanks for all the of feedback and the valuable implementation hints!
Let me be a bit more specific on the reason for my question:
The red LED shall already clearly visibly light up with a few mA (maybe 5mA, maybe less?) of current, that's what I meant by "sensitivity". I don't know which specific type this may provide and I don't have LEDs in the drawer at all to try out which one would fit and therefore would like suggestions what to buy. Maybe that's possible...
So, the red LED is the more ecritical one, as it indicates failure drivern by the output transistor of an SFH6206 optocoupler and should visibly light up with as little current into the OC Diode input as possible. To achieve that I have made provisions on the board for an additional optional transistor forming a darlington with the output NPN of the OC if necessary. The green LED indicates OK and could be adjusted in brightness as advised, i.e. less critical.
Hope the above makes good sense of what I'd like to do...
Greetings,
Winfried
You are worrying over nothing 🙂
Something like this. I would get an LED with a red package rather than a clear package as these are probably visually more suited to what you want.
LED's are so cheap you could get a couple of different efficiencies and see how you get on.
Less bright:
https://cpc.farnell.com/multicomp-pro/mcl053pd/led-5mm-36-hi-red/dp/SC15412
Brighter:
https://cpc.farnell.com/kingbright/l-53id-5v/led-5mm-he-red/dp/SC07648
Very bright:
https://cpc.farnell.com/kingbright/l-53srd-5v/led-5mm-ultra-red/dp/SC07651
And remember its all relative. The least bright of these is magnitudes brighter than LED's used back in the 80's as power on indicators. Those often had figures in 1mcd and below range and they needed 20 or 30 milliamps. Even a cheapo LED will achieve that today with 1 or 2 milliamps.
Something like this. I would get an LED with a red package rather than a clear package as these are probably visually more suited to what you want.
LED's are so cheap you could get a couple of different efficiencies and see how you get on.
Less bright:
https://cpc.farnell.com/multicomp-pro/mcl053pd/led-5mm-36-hi-red/dp/SC15412
Brighter:
https://cpc.farnell.com/kingbright/l-53id-5v/led-5mm-he-red/dp/SC07648
Very bright:
https://cpc.farnell.com/kingbright/l-53srd-5v/led-5mm-ultra-red/dp/SC07651
And remember its all relative. The least bright of these is magnitudes brighter than LED's used back in the 80's as power on indicators. Those often had figures in 1mcd and below range and they needed 20 or 30 milliamps. Even a cheapo LED will achieve that today with 1 or 2 milliamps.
Mooly,
your last answer is just what was needed because of my limited LED experience and wanting to avoid unnecessary difficulty 🙄 😉. I'll look at the products and make my choice, probably go for the "brighter" category, the rest is potentionmeter adjustment then.
Thanks a lot!
Winfried
your last answer is just what was needed because of my limited LED experience and wanting to avoid unnecessary difficulty 🙄 😉. I'll look at the products and make my choice, probably go for the "brighter" category, the rest is potentionmeter adjustment then.
Thanks a lot!
Winfried
A pot is fine as a way of determining the required current but please also add a series resistor resistor as wiseoldtech suggested. Its so easy to turn the pot to far and burn up both the pot and LED.
No worries... 😀
@OldDIY
The application is on a bord surface for diagnosis purpose in case of a PowerAmp board problem and not permanently visible from the outside. Mounting diffusor(s) is therefore not practical in my application case.
Regards,
Winfried
One thing to remember with LEDs is that the brightness in mcd is a measure of the light emitted into a given conical angle (defined as steradians). You need to check the angle of illumination along with the mcd rating. If you want to see the light over a wide angle you will need to check the angle is wide enough for your requirements. High mcd figures might be obtained with a narrow (10 degree, say) angle. Given equal angles, equal mcd should give the same apparent luminosity.
The fundamental light output should be stated in lumens, but the relationship is that lumens=candela*2pi(1-cos(theta/2)) where theta is the emission angle.
The fundamental light output should be stated in lumens, but the relationship is that lumens=candela*2pi(1-cos(theta/2)) where theta is the emission angle.